The mechanisms whereby the blood-brain barrier (BBB) to proteins, e.g. horse-radish peroxidase (HRP) is opened in response to hyperosmotic agents are being scrutinized with a tracer smaller than HRP or colloidal lanthanum. The inference that the openings may be parajunctional channels through the endothelium rests on our demonstration that vesicular transfer cannot account for the escape of protein; the exudate pattern is the same before fixation, which halts vesicular movement. In order to see whether junctions or channels are implicated, a tracer not much larger than the hydrated sodium ion, i.e. ionic lanthanum is to be used. The threshold concentration of 1.6 M arabinose that has been used for rats and of 2.0 M urea for rabbits will be used on rat specimens only. The disadvantage of ionic lanthanum is that it cannot be detected by light microscopy. The great advantage is that the tracer, being ionic, is more interesting physiologically and the results may be applicable to other di- and tri-valent cations. About 2 hours after arabinose administration, 90% of the barrier has been re-established. We shall examine these reversibly "closed" vessels along with the few that are still permeable.